Aromatic aldehydes have a wide range of usage since they have aldehyde groups having high reactivity. Especially, terephthalaldehyde having two aldehyde groups at para position has been brought to attention as raw material for medicinal products, agrochemicals, pigments, liquid crystal polymers, or plastic having heat resistance.
Dehydration method of intermediate prepared by chlorination of p-xylene, and hydrogenation method of dimethyl terephthalate are conventional methods for the preparation of terephthalaldehyde known to the art. However, these conventional methods are not suitable for economical mass production since the processes are complicated and should be carried out under high pressure and non-environment-friendly conditions.
In order to solve these problems, there has been a continuous study for mass production of terephthalaldehyde by gas-phase-oxidizing p-xylene with molecular oxygen.
For example, Japanese Patent Publication No. 47-002086 discloses a complex oxide catalyst having the ratio range of W:Mo of from 1:1 to 20:1. And, Japanese Patent Publication No. 48-047830 discloses a catalyst comprising V and Rb or Cs. U.S. Pat. No. 3,845,137 discloses a catalyst consisting of two components, W and Mo, and one or more components selected from the group consisting of Ca, Ba, Ti, Zr, Hf, Tl, Nb, Zn, and Sn. Also, U.S. Pat. No. 4,017,547 discloses a catalyst consisting of Mo oxide, W oxide or silicotungstic acid and Bi oxide. However, the industrial practical use of these catalysts has been limited due to the low selectivity and yield of terephthalaldehyde.
Also, U.S. Pat. No. 5,324,702 discloses a catalyst comprising a first component selected from the group consisting of Fe, Zn, Zr, Nb, In, Sn, Sb, Ce and Bi, and a second component selected from the group consisting of V, Mo and W, wherein the first and second components are distributed on a deboronized borosilicate crystal molecular sieve by chemical vapor deposition (CVD). This catalyst shows relatively higher conversion rate to p-xylene, and relatively higher yield of terephthalaldehyde, than conventional catalysts. However, the catalyst has a limit in increasing the selectivity for various by-products, and so it was difficult to separate and purify it.
Also, Japanese Laid-open Patent Publication No. 2001-198464 discloses a catalyst comprising W as main component; one or more essential components selected from the group consisting of P, Sb, Bi and Si; and one or more optional components selected from the group consisting of Fe, Co, Ni, Mn, Re, Cr, V, Nb, Ti, Zr, Zn, Cd, Y, La, Ce, B, Al, Tl, Sn, Mg, Ca, Sr, Ba, Li, Na, K, Rb and Cs. The catalyst can provide high yield of terephthalaldehyde almost to the extent having industrial applicability. However, the catalyst also has limits in the separation and purification since the selectivity of terephthalaldehyde is not high, compared with the high conversion rate of p-xylene. Also, the catalyst has the problems of heat stability and life span since it comprises Sb component which is sublimated and lost at high temperature.
In short, in case of using the conventional catalysts, the terephthalaldehyde's yield is low. Or, the selectivity is low even though the yield is high. Thus, the separation and purification are difficult. Also, it is difficult to prepare the catalysts to have homogeneous composition and performance since they use a complex oxide having multiple components. Further, the catalysts comprise components having low heat stability, and so have short life span, and thus their industrial practical uses are limited.
On the other hand, Korean Patent Application No. 10-2004-0089376 filed by the present inventor disclosed a single-component catalyst comprising tungsten oxide, and fire-resistant inorganic carrier as optional component. The catalyst has advantages that it can be easily homogenous and has higher selectivity and yield of terephthalaldehyde than conventional complex oxide having multiple components. However, a catalyst having higher selectivity for preparing terephthalaldehyde has been still required because the future research trend of partial oxidation reaction process field will lie in development of a catalyst which can reduce the green house gas which is a major by-product, and can increase the selectivity of terephthalaldehyde, which will be very important standards for commercialization of catalyst process in the future.